It acted as a control processor with a lot of different roles. For instance, it could handle the interface signals between an external computer and a piece of HP test equipment. Or it could process button presses and control the display. Or it could parse ASCII command strings. With an external 74181 ALU chip, it could even compute error percentages in a voltmeter.
Quick question on the post, in the paragraph on metal gate transistors there are a couple of advantages listed. The last one says, "Third, metal-gate circuitry required an additional +12 V power supply."
It's not clear to me why a +12V supply would be an advantage? Did 1974 have their equivalent of ATX supplies so that meant one less rail the PCB needed to generate compared to silicon-transistor designs?
And there is some interesting cut in the ceramic that is exposing the lead frame on the left side of the package in the picture that is showing where the -2.5V bias note was written. Do you know what that is?
As jacobr1 pointed out, it's a disadvantage of metal-gate transistors, not an advantage. Silicon-gate transistors are entirely better than metal-gate transistors. (Except now metal-gate transistors are coming back for high performance, using "high-k" metals like hafnium. But these are completely different from 1970s-era metal-gate transistors.)
For your other question, the visible metal trace on top of the IC package is a common thing with ceramic packages. I think the purpose was to ground the metal lid of the IC to protect against noise, static, etc.
If the metal strip is on the top of the ceramic and grounded it makes sense that it would be for ESD protection. Especially as they likely did hand assembly of the PCBs.
Interestingly, the $360 quoted in the article is not that far off from a modern desktop Intel chip. For $25 today you can get an entire computer in the form of a Raspberry Pi 3 A+.
It was a cheap to manufacture chip with extremely limited capabilities. We certainly still have those, but the art has advanced to where the "extremely limited" part is not as limiting as it once was.
That doesn't really mean anything, other than the fact that the technology has gotten insanely cheaper. It's much more relevant to the question to compare the relative costs to modern chips. I just thought it was interesting that we could actually do this using things relatively close to the original price points in the article.
